The present invention relates to an improved ceramic composition for preparing multilayer ceramic structures for electronic devices, and, more particularly, to a method of enhancing, i.e., improving, the sinterability of such compositions at low firing temperatures by improving the distribution of sintering flux, and especially zinc borate based flux, uniformly throughout the composition.
Multilayer ceramic capacitors (MLC's) are the most widely used form of ceramic capacitors because of their high volumetric efficiency and their small size. These capacitors are typically manufactured by co-firing, i.e., sintering a ceramic dielectric formulation and a conductive electrode material in an oxidizing atmosphere at a temperature in the range of about 1200.degree. to 1400.degree. C. Firing under these conditions, however, requires an electrode material with high melting point, good oxidation resistance at elevated temperatures, sinterability at the maturing temperature of the dielectric, and minimal tendency to interact with the dielectric at the sintering temperature. These requirements normally limit the choice of electrode materials to the noble metals platinum and palladium or to alloys of platinum, palladium or gold.
The ceramic formulation must sinter to a dense hermetic body at a temperature below the melting point of the conductor co-fired with the ceramic in the multilayer structure, e.g., below about 1150.degree. C. for a 70% Ag/30% Pd alloy and below 1083.degree. C. for Cu. In the case of copper, the structure must also be fired in a non-oxidizing atmosphere to protect the conductors from oxidation during the sintering process. To achieve a dense hermetic structure when sintering at temperatures below 1150.degree. C., fluxes can be added to the ceramic dielectric formulations. U.S. Pat. No. 4,640,905, for example, describes a manganese doped zinc borate flux, and a low firing ceramic based on barium titanate with a high dielectric constant (K) for use in MLC's with Ag/Pd conductors.
USSR Patent 692,810 describes the presence of 3 to 5 wt % of zinc borate in a barium titanate based ceramic sintered "cake" used to manufacture high frequency capacitors at a low sintering temperature. The capacitors are made by pressure forming and firing at 1020.degree. to 1080.degree. C. in an oxidising atmosphere.
U.S. Pat. No. 4,845,062 describes a zinc borate frit used as a sintering aid for a magnesium titanate based ceramic in MLC's with copper conductors.
U.S. Pat. No. 4,879,261 describes a low dielectric constant composition based on silicon dioxide with zinc borate frits used for making multilayer substrates with copper conductors. The substrates can be used for mounting active electronic components such as integrated circuits.
Glass compositions in which zinc borate is a component in admixtures of numerous metal oxides are documented in the prior art, however, coating ceramic particles prior to being sintered to form monolithic electronic components is not revealed.
U.S. Pat. No. 4,610,968 and U.S. Pat. No. 4,610,971 describe a ceramic dielectric composition capable of being sintered at a sufficiently low temperature to enable the use of a low cost base metal as the electrode material in the fabrication of capacitors. The dielectric composition is made by a ball milling process described in U.S. Pat. No. 4,626,396.
The addition of fluxes to ceramic powders has a tendency to degrade the dielectric properties of the sintered dielectric, e.g., producing a lower K, lower mechanical strength and a higher than desired dissipation factor. Thus, submicron ceramic powders are used to increase sinterability and allow for minimal amounts of fluxing additives. The particle size of the flux should be very small, i.e., smaller than the size of the ceramic powder. For advanced dielectric formulations, the ceramic powder particle size may be substantially below 1 micron, requiring a flux particle size of a few tenths of a micron. In general, it is difficult to prepare powders of the flux composition in which the particle size is significantly below one micron. This is due to the difficulty of milling glassy materials to yield submicron particles and the tendency of such particles to agglomerate into larger particles. Thus, fluxes described in the prior art tend to have a particle size of about one micron or larger. Even if fluxes could be prepared with smaller particle size, it may be impossible to achieve uniform distribution of the flux particles among the particles of the ceramic powder because of a tendency of very fine particles to flocculate in suspensions with polymer and solvent.
The difficulty of achieving a uniform distribution of the flux in low firing dielectric compositions has been overcome by the present invention in which the ceramic particles are coated with flux by special deposition techniques, and the coating methods can be applied generally to a variety of ceramic powders without excessive removal of ions from the surface of the powder particles during the coating process.